Parasitic helminth infections in animals, including humans, are typically treated by chemical drugs, because there are essentially no efficacious vaccines available. One disadvantage with chemical drugs is that they must be administered often. For example, dogs susceptible to heartworm are typically treated monthly to maintain protective drug levels. Repeated administration of drugs to treat parasitic helminth infections, however, often leads to the development of resistant strains that no longer respond to treatment. Furthermore, many of the chemical drugs are harmful to the animals being treated, and as larger doses become required due to the build up of resistance, the side effects become even greater.
It is particularly difficult to develop vaccines against parasitic helminth infections both because of the complexity of the parasite's life cycle and because, while administration of parasites or parasite antigens can lead to the production of a significant antibody response, the immune response is typically not sufficient to protect the animal against infection.
As for most parasites, the life cycle of Dirofilaria immitis, the helminth that causes heartworm, includes a variety of life forms, each of which presents different targets, and challenges, for immunization. Adult forms of the parasite are quite large and preferentially inhabit the heart and pulmonary arteries of an animal. Males worms are typically about 12 cm (centimeters) to about 20 cm long and about 0.7 mm to about 0.9 mm wide; female worms are about 25 cm to about 31 cm long and about 1.0 to about 1.3 mm wide. Sexually mature adults, after mating, produce microfilariae which are only about 300 .mu.m (micrometers) long and about 7 .mu.m wide. The microfilariae traverse capillary beds and circulate in the vascular system of the dog in concentrations of about 10.sup.3 to about 10.sup.5 microfilariae per ml of blood. One method of demonstrating infection in the dog is to detect the circulating microfilariae.
If the dog is maintained in an insect-free environment, the life cycle of the parasite cannot progress. However, when microfilariae are ingested by the female mosquito during blood feeding on an infected dog, subsequent development of the microfilariae into larvae occurs in the mosquito. The microfilariae go through two larval stages (L1 and L2) and finally become mature third stage larvae (L3) of about 1.1 mm length, which can then be transmitted back to the dog through the bite of the mosquito. It is this L3 stage, therefore, that accounts for the initial infection. As early as three days after infection, the L3 molt to the fourth larval (L4) stage, and subsequently to the fifth stage, or immature adults. The immature adults migrate to the heart and pulmonary arteries, where they mature and reproduce, thus producing the microfilariae in the blood. "Occult" infection with heartworm in dogs is defined as that wherein no microfilariae can be detected, but the existence of the adult heartworms can be determined through thoracic examination.
Heartworm not only is a major problem in dogs, which typically cannot even develop immunity upon infection (i.e., dogs can become reinfected even after being cured by chemotherapy), but is also becoming increasingly widespread in other companion animals, such as cats and ferrets. Heartworm infections have also been reported in humans. Other parasitic helminthic infections are also widespread, and all require better treatment, including a preventative vaccine program.
Although many investigators have tried to develop vaccines based on specific antigens, it is well understood that the ability of an antigen to stimulate antibody production does not necessarily correlate with the ability of the antigen to stimulate an immune response capable of protecting an animal from infection, particularly in the case of parasitic helminths. A large number of materials are immunogenic and produce sera which test positive in immunoassays for ability to react with the immunizing antigen, but which fail to protect the hosts against infection. Antibodies which neutralize the infective agent in in vitro assays are much more likely to protect against challenge in vivo. Accordingly, the use of serum simply resulting from immunization or from infection by a parasitic helminth to screen for candidate vaccines does not provide sufficient specificity to identify protective immunogens. On the other hand, serum or other components of blood from immunized animals which is demonstrably protective against infection would contain antibodies, cells, or other factors that could selectively bind to potential antigens that, if used as therapeutic compositions, would elicit immune responses that protect against challenge.
In most infectious diseases, particularly those such as parasitic infections that have long and complex development courses, it is difficult to verify the protective effect of serum or T-cells from exposed animals for use as a screening reagent. First, verification of protection against challenge is tedious, since the host animal would first have to be challenged with the infectious agent and shown to be protected before it could be shown that antibody components of serum, for example, could be used as a screen. The definition of protection under such a regimen is often complex. Second, even if a protective effect against challenge is shown, it is not clear to what components of the immune system the protection is due. The protective effect could be due to antibodies, cells, mediators of the immune system or to combinations thereof. Thus, although this method of obtaining the screening reagent is sometimes used, it is time-consuming and does not permit identification of protective components.
A method to determine the effectiveness of in vivo immunization protocols includes implanting diffusion chambers containing infectious agents into immunized animals and determining the effects of such immunizations on the implanted infectious agents. Grieve, et al., 1988, Am. J. Trop. Med. Hyg. 39, p. 373-379, for example, report that dogs which had been immunized against Dirofilaria immitis infection were supplied diffusion chambers containing infective larvae. The larvae in the chambers could then be evaluated for the effect of the previous immunizations. Abraham, et al., 1988 J. Parasit. 74, p. 275-282, report that mice which had been immunized with L3 were supplied diffusion chambers containing D. immitis third-stage larvae, and the effects on these larvae were used to determine the possible immunity of the mice putatively developed by such immunization. Thus, the papers disclose that implantation of diffusion chambers containing the infectious agent into an immunized animal provides a convenient assessment of the effectiveness of certain directly administered active immunization protocols, but do not describe the use of such chambers to monitor passively transferred protective effects of selected fractions of a target host bloodstream.
Protection against parasitic helminth infections is difficult to achieve because, as heretofore stated, the complexity of the parasitic infection makes the choice of a candidate immunogen for vaccination very difficult. Even naturally conferred immunity cannot be assured to exist, as dogs with previous or existing infections with D. immitis can be reinfected (see, for example, Grieve et al., 1983, Epidemiologic Reviews 5, p. 220-246). However, this review also reports that there is some evidence of a naturally occurring protective immune response, which apparently limits the population of mature worms in infected dogs.
Furthermore, it has been possible to induce protective immunity artificially. Wong, et al., 1974, Exp. Parasitol. 35, p. 465-474, reported the immunization of dogs with radiation-attenuated infective larvae. The dogs were protected to varying degrees upon challenge. Blair, et al., 1982 in Fifth International Congress of Parasitology, Toronto, Canada, reported successful immunization by infecting the dogs and terminating the infection at the fourth larval stage by chemotherapy.
Grieve, 1989, Proc. Heartworm Symp., p. 187-190, reviewed the status of attempts to produce vaccines against heartworm in dogs. This report summarizes the use of infective larvae implanted in an inert diffusion chamber which permits the influx of cells and/or serum from the host and outflow of parasite material from the chamber to assess the effectiveness of inoculation protocols in both dogs and mice. The use of immunization with infective larvae was demonstrated to be partially effective in protection against subsequent challenge.
An alternative approach to finding, for example, a heartworm vaccine has been to attempt to identify prominent antigens in the infective stage of D. immitis. Philipp, et al., 1986, J. Immunol. 136, p. 2621-2627, reports a 35-kilodalton (kD) major surface antigen of D. immitis third stage larvae which was capable of immunoprecipitation with sera from dogs carrying an occult experimental D. immitis infection or with sera from dogs immunized by irradiated third stage larvae. In addition, this group reported (Davis, et al., 1988, Abstract 404, 37th Annual Meeting, Am. Soc. Trop. Med. Hyg.) three major surface proteins of the L4 having molecular weights of 150 kD, 52 kD, and 25 kD. The 25 kD molecule seemed unique to L4 larvae.
Ibrahim, et al., 1989, Parasitol. 99, p. 89-97, using D. immitis L3 larvae labeled with .sup.125 I, showed that a 35 kD and 6 kD component were shed into the culture medium by developing parasites. They further showed that antibodies from immunized rabbits and infected dogs immunoprecipitated the 35 kD, but not the 6 kD, component.
Scott, et al, 1990, Acta Tropica 47, p. 339-353, reported characterization of the surface-associated molecules of D. immitis L2, L3, and L4 by radiolabeling techniques and SDS-PAGE. They found major labeled components of 35 kD and 6 kD in extracts from iodine-labeled L2 and L3; lactoperoxidase-catalyzed labeling revealed components of apparent molecular weights 66 kD, 48 kD, 25 kD, 16.5 kD, and 12 kD. Iodine labeling of surface-associated molecules of L4 gave molecules of apparent molecular weights of 57 kD, 40 kD, 25 kD, 12 kD, and 10 kD; lactoperoxidase-catalyzed labeling showed additional bands of 45 kD, 43 kD, and 3 kD. However, these antigens were identified using uncharacterized serum sources.
Other approaches to obtaining vaccines against parasites in general have focused on the production of neutralizing antibodies. For example, both in vitro studies by Tanner, et al., 1981, Trans. Roy. Soc. Trop. Med. Hyg. 75, p. 173-174 and by Sim et al., 1982, Trans. Roy. Soc. Trop. Med. Hyg. 76, p. 362-370, and in vivo studies by Parab et al., 1988, Immunol. 64, p. 169-174, have demonstrated that antibodies are effective alone or with other immune components in killing filarial L3 from Dipetalonema (Acanthocheilonema) viteae or Brugia malayi. Furthermore, passive immunity to Schistosoma mansoni has been transferred from immune rats or humans to normal mice (see, for example, Sher, et al., 1975, Parasitol. 70, p. 347-357; Jwo et al., 1989, Am. J. Trop. Med. Hyg. 41, p. 553-562). None of these studies involved the use of an in vivo assay to determine the ability of serum, or cellular, components to be a useful screening tool for identifying protective antigens. Neither has any of these studies yet identified an effective vaccine.